1. Technical Field
The disclosed embodiments relate to local oscillators and mixers.
2. Background Information
FIG. 1 (Prior Art) is a very simplified high level block diagram of circuitry of a cellular telephone 1. Cellular telephone 1 includes an antenna 2, several discrete components, and several integrated circuits including a radio frequency (RF) transceiver integrated circuit 3 and a digital baseband integrated circuit 4. Digital baseband integrated circuit 4 includes primarily digital circuitry and includes a digital processor. RF transceiver integrated circuit 3 includes primarily analog circuitry. RF transceiver integrated circuit 3 is called a “transceiver” because it includes a transmitter as well as a receiver.
FIG. 2 (Prior Art) is a more detailed diagram of the RF transceiver integrated circuit 3 of FIG. 1. The receiver includes what is called a “receive chain” 5 as well as a local oscillator (LO) 6. When the cellular telephone 1 is receiving, a high frequency RF signal 7 is received on antenna 2. Information from signal 7 passes through the receive chain 5 to the digital baseband integrated circuit 4. If the cellular telephone is transmitting, then information to be transmitted is converted into analog form by a digital-to-analog converter in the digital baseband integrated circuit 4. Baseband filter 8 in transmit chain 9 filters out digital noise. Mixer block 10 up-converts the low frequency signal from baseband filter 8 into a high frequency signal. Driver amplifier 11 and power amplifier 12 amplify the high frequency signal to drive antenna 2 so that high frequency RF signal 13 is transmitted from antenna 2.
The cellular telephone 1 of FIGS. 1 and 2 may be a type of cellular telephone that is sometimes referred to as a “3G” cellular telephone. The 3G cellular telephone can be used to transfer data in addition to communicating speech conversations. An example of transferring data is browsing the internet. A typical 3G cellular telephone communication standard (for example, Wideband Code-Division Multiple Access (WCDMA)) requires that the cellular telephone be able to control the magnitude of RF output power with which the cellular telephone transmits to a base station. Accordingly, digital baseband integrated circuit 4 within cellular telephone 1 is able control the RF transceiver integrated circuit 3 such that the power of the transmitted RF signal 13 has a specified one of a plurality of discrete “steps” or “levels” of output power. The 3G cellular telephone standard generally specifies the lowest output power level that may be used, specifies the maximum output power level that may be used, and specifies a number of steps of selectable output power levels in between the minimum and maximum output power levels. The power levels are be repeatable over all phones manufactured.
In the cellular telephone 1 of FIGS. 1 and 2, transmit output power is a function of the operation of baseband filter 8, mixer block 10, and driver amplifier 11. The digital baseband integrated circuit 4 can therefore control the magnitude of transmit output power by controlling the baseband filter, mixer block, and/or driver amplifier using control lines 14-16.
FIG. 3 (Prior Art) is a more detailed diagram of a portion of FIG. 2. If, for example, baseband filter 8 is outputting a 1 MHz baseband signal 17, and it is desired to up-convert this baseband signal in frequency into a 901 MHz high frequency output signal 18, then local oscillator 19 is controlled to output a 900 MHz local oscillator signal 20. Baseband processor integrated circuit 4 controls the frequency of the local oscillator signal 20 and is therefore able to control the frequency of the high frequency output signal 18 that drives driver amplifier 11.
One operational parameter of the circuitry of FIG. 3 is the accuracy with which the output power of signal 18 can be controlled. Mixer block 10 actually includes multiple mixers 21-23 that are connected together in parallel as shown. The size of a mixer determines the power that the mixer can output. The various mixers 21-23 are sized in a binary weighted manner so that each successive mixer is double the size of the prior mixer. The output powers of all the mixers 21-23 are combined in a primary winding 24 of a transformer 25 so that driver amplifier 11 is driven by the combined power of all the mixers that are enabled. Due to the binary weighting of the sizes of mixers 21-23, the power of signal 18 can be set to have an output power in any one of eight power steps 0X to 7X by enabling and disabling selected ones of mixers 21-23. Every doubling of the output power increases the output power by 6 dB.
Mixers 21-23 are realized as separate mixers on the RF transceiver integrated circuit 3 where each successive mixer is twice the size of the prior mixer. It would be expected that a second mixer that is twice the size of a first mixer would have an output power that is twice as great as the output power of the smaller first mixer. In reality, however, there may be somewhat complex high frequency parasitic effects due to differences in the physical structures that make up the various mixers and due to interactions between adjacent physical structures. The impact of these parasitic effects on mixer output power generally does not scale with mixer size. Accordingly, the enabling and disabling of mixers whose sizes are binary weighted does not always result in adequate accuracy of the power of the output power steps.
A second operational parameter of the circuitry of FIG. 3 is local oscillator leakage. In the illustrated example in which the baseband signal 17 is a pure 1 MHz signal and the local oscillator signal 20 is a pure 900 MHz signal, then the high frequency output signal 18 is a pure 901 MHz signal. In an ideal case, the output of mixer block 10 in this case will not include any of the 1 MHz baseband signal and will not include any of the 900 MHz local oscillator signal. In reality, however, mixer block 10 outputs the desired 901 MHz signal but also outputs some amount of a signal at 900 MHz. The 900 MHz signal is said to be “leakage” from the local oscillator. Once the 900 MHz signal gets to the transformer 25, the 900 MHz signal is amplified by driver amplifier 11 and power amplifier 12 and is transmitted from antenna 2. This is undesirable. There is a relationship between the amount of power with which local oscillator 19 drives mixer block 10, and the loading on the output of the local oscillator due to which ones of the mixers 21-23 within mixer block 10 are enabled. If transmitter output power is to be reduced and the combined size of the enabled mixers in mixer block 10 is reduced, then local oscillator leakage may increase as a proportion of the desired high frequency signal 18 because the local oscillator 19 continues to drive the mixer block 10 with the same power even though the loading presented by the mixer block 10 has been reduced. Moreover, there is leakage through mixers that are not enabled, so the amount of leakage does not track well with the output power supplied by the mixers. Cellular telephone standards generally specify that the power of local oscillator leakage be below the output power of the RF output signal by a certain amount. When digital baseband integrated circuit 4 controls the RF transceiver integrated circuit 3 to reduce the output power of the desired signal, local oscillator leakage should also be reduced a proportional amount in order to remain within the requirements of the cellular telephone standard. In the circuit topology of FIG. 3, however, the reduction in local oscillator leakage may not track with the reduction in output power of the desired output signal 18 in a suitable manner.